In recent years, a sheet press molding method has been drawing attention as a method of molding thin and large-area products, and has been used for, for example, molding of fuel cell separators. Hereinafter, the sheet press molding method will be described with reference to an example of a method of manufacturing a fuel cell separator.
A fuel cell is a clean power generation apparatus that generates power by the reverse reaction of water electrolysis, in which hydrogen and oxygen are used, and produces no emissions other than water, and is thus gaining attention from the viewpoint of environmental issues and energy issues. Fuel cells are classified into several types depending on the type of an electrolyte being used. Among fuel cells, a solid polymer electrolyte fuel cell operating at a low temperature seems most promising for automobiles or consumer use. Generally, this fuel cell has a basic unit of a single cell including a solid polymer membrane acting as a solid polymer electrolyte, a membrane-electrode assembly (MEA) formed by integrating gas diffusion electrodes that support a pair of catalysts interposing the solid polymer membrane, and a separator interposed from the outside of the MEA so as to separate a fuel gas and an oxidized gas. In addition, high-output power generation can be attained by stacking a number of the single cells in a fuel cell.
Gas flow channels (grooves) are provided on the surface of the separator composing the above fuel cell, which is in contact with the MEA, to supply a reactant gas to the gas diffusion electrode surfaces and carry away produced gas or excessive gas. Using these gas flow channels, hydrogen, which is a fuel, is supplied to one gas diffusion electrode side, an oxidant gas, such as oxygen or air, is supplied to the other gas diffusion electrode side, and an external load circuit is connected between the two gas diffusion electrodes, making it possible for a device having the above configuration to be operated as a fuel cell.
Therefore, the separator needs to be excellent in terms of thermal conductivity, strength, and the like in addition to being sufficiently highly gas impermeable to completely separate the gases and being highly electrically conductive enough to reduce internal resistance. In addition, since the fuel cell is composed by stacking a number of single cells as described above, there is demand for a decrease in the weight and thickness of the separator.
Thus far, metallic materials and carbonaceous materials have been studied as a material for the fuel cell separator in order to meet the demand. Metallic materials are excellent in terms of mechanical properties, and thus have an advantage of an ability to obtain a thin separator or a high electrical conductivity. However, metallic materials have a large specific gravity, and are insufficient in terms of corrosion resistance as well.
Carbonaceous materials are light and highly electrically conductive materials that are excellent in terms of thermal conductivity, strength, and the like, and studies are underway regarding thin molding techniques or mass production of carbonaceous materials.
For example, PTL 1 discloses a cumbersome process in which a binder is added to carbonaceous powder, the two are heated, mixed and then subjected to cold isostatic pressing (CIP), an isotropic graphite material obtained by firing and graphitizing the above mixture is impregnated with a thermosetting resin and subjected to a curing treatment, and then the material is subjected to slice machining, thereby producing a separator for a solid polymer electrolyte fuel cell. In addition, PTL 2 discloses a method of manufacturing a thin carbon plate in which paper including carbon powder or carbon fiber is impregnated with a thermosetting resin, and plural sheets of the paper are laminated, pressed, cured, and fired. In addition, PTL 3 discloses a method of manufacturing a fuel cell separator in which a phenol resin molding material is injection-molded using a mold, and the obtained molded product is fired.
However, fired materials are used as a material for the fuel cell separator in the techniques described in PTL 1 to 3. Fired materials exhibit a high electrical conductivity and high thermal resistance, but have a problem of a tendency of brittle fracture or a problem of poor productivity due to the long time necessary for firing. Furthermore, as described in PTL 1, since materials requiring cutting machining, such as slice machining, in a process of manufacturing a separator become more deficient in productivity, and costs become high, the materials have many difficulties becoming widespread in the future.
As a technique to solve this problem, a method of manufacturing a fuel cell separator by, for example, performing sheet press molding in which a sheet-shaped material including an electrically conductive carbonaceous material is press-molded is suggested. Such a method is excellent in terms of productivity, and, in particular, is preferably used when manufacturing thin separators.
However, when a fuel cell separator is manufactured by press molding of a sheet-shaped material including a carbonaceous material, since variation in the bulk density among obtained fuel cell separators is large, there is a disadvantage of large variation in electrical conductivity, mechanical strength, airtightness, and the like.
As a technique to solve this problem, for example, PTL 4 discloses a method of manufacturing a fuel cell separator in which a first sheet made of a flexible graphite sheet and a second sheet made of a flexible graphite sheet, whose portions corresponding to flow channels are removed, are laminated, and press molding is performed on the laminated sheets, thereby forming flow channels and through-holes, and achieving an increase in the bulk density of the peripheral areas. In addition, PTL 5 discloses a technique in which protrudings and recesses are formed on the surface of an expanded graphite sheet so that the density difference of expanded graphite becomes less than 30%, and then a fuel cell separator is formed using a press having a shape matching the recess and protrusion-shaped portion from an expanded graphite compact formed into a predetermined recess and protrusion form.
However, as a material used for the fuel cell separator, a flexible graphite sheet is used in PTL 4, and an expanded graphite sheet is used in PTL 5. Since the flexible graphite sheet and the expanded graphite sheet are porous, the two have an intrinsic problem of the bulk density difference in portions of the obtained compact having different thicknesses when a recess and protrusion pattern is press-molded using the above sheets. Therefore, there has been a demand for a further decrease in the variation of characteristics resulting from the variation of the bulk density among fuel cell separators. In addition, the flexible graphite sheet and the expanded graphite sheet have a disadvantage of a tendency of having surface defects, such as cracking and blisters, being created by press molding.
As a technique to solve this problem, there is a method in which a molded product is obtained by press molding of a sheet-shaped material including a resin composition and a carbonaceous material. For example, PTL 6 discloses a fuel cell separator made of a cured article obtained by curing with a compression molding machine an uncured sheet that is molded using a electrically conductive curable resin composition including a curable resin composition, including elastomer having a Mooney viscosity (ML1+4 (100° C.)) of 25 or higher, and a carbonaceous material in a mass ratio of 70:30 to 5:95.
As such, when a material including a resin composition and a carbonaceous material is used as the sheet-shaped material, the variation in the characteristics of molded products obtained by performing press molding can be reduced, and defects are also prevented during press molding in comparison to the case of using the flexible graphite sheet and the expanded graphite sheet.